The rapid expansion of digital signal processing hardware and software has dramatically increased the demand for high-speed and high-resolution analog-to-digital converters. The most popular techniques for analog-to-digital conversion to date include successive approximation, sigma-delta, sub-ranging, flash, and time-interleaving.
The advancement of analog-to-digital converter technology involves a trade-off between sampling rate in samples per second (Sa/s) and resolution in bits. Sigma-delta converters currently provide high resolution (20 bits) at relatively low sampling rates (10 kSa/s) while time-interleaved converters provide low resolution (8 bits) at high sampling rates (8 GSa/s). The remaining techniques including successive approximation (100 kSa/s at 16 bits resolution), subranging (1 MSa/s at 14 bits resolution), and flash (100 MSa/s at 10 bits resolution), span the range of sampling rate, resolution, and cost between the time-interleaving and sigma-delta techniques.
Recently, manufacturers began studying and perfecting time-interleaved analog-to-digital conversion. In this technique, a bank of time-multiplexed analog-to-digital converters with low sampling rates are combined to realize a single high-speed, high-resolution analog-to-digital converter. The primary drawback of time-interleaved analog-to-digital conversion is that linearity errors and mismatches between each of the converters in the bank are compounded across the full bandwidth of the device, limiting the resolution of the system. Time-interleaving is also prone to timing errors because it requires very accurate interleaved clock signals which limits the speed and resolution of the system and which introduces harmonic distortion. In addition, a voltage offset between two converters in the bank may cause the converters to digitize the same input voltage to different codes. Since the converters output data sequentially, the output exhibits an error signal having a period equivalent to the sample period of a single converter, but the frequency content can increase because each cycle of the error contains a number of offset errors equivalent to the number of converters in the bank. Therefore, this source of error introduces harmonic distortion spurs that limit the effective resolution of the system. A difference in voltage gain between two converters also causes each converter to digitize the same input voltage to a different code. The magnitude of the error grows as the input voltage grows. Overcoming these fundamental limitations is difficult because it requires reduction of error for each converter. The gain and offset of an individual converter can be adjusted with external resistors and the remaining harmonic distortion can be reduced with dynamic compensation techniques. However, the remaining linearity errors of each converter are still compounded across the full bandwidth of the system.
Discrete-time Quaudrature Mirror Filter (QMF) banks have been employed in analog-to-digital conversion techniques as described in:
Antonio Petraglia and Sanjit K. Mitra, "High-Speed A/D Conversion Incorporating a QMF Bank," IEEE Transactions on Instrumentation and Measurement, 41(3):427-431 (Jun., 1992).
In this system, discrete-time switched-capacitor analysis filters decompose the wideband input signal into several contiguous frequency subbands. An individual analog-to-digital converter is assigned to each subband. All converters are driven by a common clock. The subbands are allocated certain quantization bits thereby allowing separate control of resolution in each subband. Discre-te-time finite impulse response synthesis filters reconstruct each subband, cancelling the aliasing caused by other subbands. Therefore, the errors associated with a particular subband are not compounded with errors of other subbands as in time-interleaved analog-to-digital conversion. In addition, the discrete-time QMF bank approach does not require extremely accurate time-skewed clock signals.
The primary disadvantage of the discrete-time QMF bank analog-to-digital converter is that the switched-capacitors introduce switching noise that can limit the signal-to-noise ratio and thereby limit the resolution and speed of the system. Typical switched-capacitor filters have a signal-to-noise ratio of about 85 dB and are limited to sampling rates of about 150 kSa/sec. This limits the bandwidth of the discrete-time QMF bank.